1. Field of the Invention:
This invention relates to compositions and methods for acidizing subterranean formations in the vicinity of a well borehole. More specifically, the invention relates to additive compositions useful in preventing the undesirable effects associated with solubilized metal ions in the fluids used in well acidizing, such as the formation of asphaltene sludge.
2. Description of the Prior Art:
Acidization of oil-bearing strata is a well known method of increasing oil production. The main constituent of such acid treating compositions is the acid, usually hydrochloric acid, hydrofluoric acid, formic acid, acetic acid, citric acid, and mixtures thereof.
As the acid is moved from its manufacturing source to the wellbore and into the geological formation, it comes into contact with metals in various forms. Although commercial hydrochloric acid is available at a concentration of about 30 to 40 percent by weight, concentrated acid is generally diluted with water to about 15 percent for most acidizing jobs. However, acid varying in strength from about 5 to about 30 percent HCl has been used in the past. To prevent metal ions (the most common being iron) from becoming dissolved in the acid, the acid is stored in non-reactive or lined storage vessels prior to use. However, as the treatment fluid containing the acid is pumped through surface pumps and through downhole equipment in the wellbore, iron and other metals can become solubilized in the fluid.
In evaluating returning fluids from over 60 wells, we have found that concentrations of 25-35 thousand parts per million (ppm) iron had become solubilized in the initial treatment acid. We have also concluded that in performing an acid job on a well, that the treatment acid can solubilize 1,000 parts per million of total iron going through 700 meters of tubing and pumping equipment. This total iron is made up of approximately 200 ppm ferric ion (Fe+3) and 800 ppm ferrous ion (Fe+2). When treatment acid with 1000 ppm total solubilized iron contacts many crude oils with a reactive asphaltene component, an acid or oil insoluble sludge can result.
Formation damage resulting from the precipitation of asphaltene deposits from crude oil has been recognized in the petroleum industry for many years. The asphaltene fraction of the crude oil is that portion which is precipitated by the addition of a low molecular weight paraffin solvent such as N-pentane. Asphaltene particles are composed of condensed aromatic ring structures containing a significant number of heteroatoms (nitrogen, oxygen, sulfur). These particles are very reactive when put in a low pH environment. Asphaltenes in crude oil in low pH (acid) will extract metals from a mineral acid, i.e. HCl. Ferric chloride has been used to crosslink asphaltenes in the asphalt industry for many years in order to make road pavement and is another example of this phenomenon.
In order to lessen the problems associated with acid corrosion, most well acidizing treatments include corrosion inhibitors which retard or prevent the solubilization of metal ions in the treatment fluid. Those skilled in the art recognize certain criteria which a corrosion inhibitor must meet in order to be acceptable. The inhibitor should provide a high degree of protection (i.e. low corrosion rate) in the acid of choice. The inhibitor should be stable enough to provide adequate corrosion protection under any and all well treatment conditions. The inhibitor should have dispersability characteristics which allow it to go where it is needed and in the proper amounts. The inhibitor should be compatible with all acid additives to be employed in the treatment and should not cause any adverse secondary effects. It has also become apparent that the inhibitor must not effect acid injectivity and cleanup after the treatment and that the inhibitor should not contain insoluble residues that produce formation damage at bottom hole temperatures.
The classification system set out by the American Petroleum Institute in API Bulletin D-15 first ed. 1973 establishes criteria for determining the effectiveness of a corrosion inhibitors in preventing the formation of asphaltene sludge:
CLASS I - The inhibited acid solution is free and clear of any evidence of oiling emulsion, agglomeration of particles, haze, or any liquid/liquid or liquid/solid phase separation.
CLASS II - The inhibited acid solution is not clear and has a definite haze or turbidity. The solution is uniform and the inhibitor is evenly distributed in the acid phase and stable for the time reported.
CLASS III - The inhibited acid solution has definite phase separation, viz, oiling out, agglomeration, or other liquid/liquid or liquid/solid phase separation.
It is an object of the present invention to provide a corrosion inhibitor for a well acidizing treatment fluid which protects the metal equipment through which the fluid passes in the treatment process.
Another object of the invention is the provision of such a corrosion inhibitor which is stable, acid soluble and compatible with various acid additives.
Another object of the invention is to provide a corrosion inhibitor which does not produce sludge by reacting with the asphaltene component in the insitu crude oil.